The application of the rapid prototyping process is well-known as state-of-the-art technology in the production of casting moulds or casting patterns without the use of tools.
A rapid prototyping process to configure casting patterns in particular is already familiar from DE 198 53 834 A1, for example. In this process untreated particulate material, such as quartz sand, is applied on a building platform in a thin layer. Then with the aid of a spray appliance a binding agent is sprayed onto the entire particulate material as thinly as possible. Subsequently a curing agent is applied to selected areas, as a result of which designated areas of the particulate material are solidified. After several repetitions of this process an individually moulded body can be produced from the bonded particulate material. This body is initially embedded in the surrounding unbonded particulate material and can be removed from the bed of particulate material after the construction process has been completed.
If, for example, in this type of rapid prototyping process a quartz sand is used as particulate material and a furan resin as binding agent, with the aid of a sulphurous acid used as curing agent a casting mould can be produced, which is made of the materials usually used in the mould production process and therefore consists of materials known to the expert.
With such rapid prototyping processes, as already described, first the particulate material, then the binding agent and thereafter the curing agent must be applied. This requires a threefold application of materials for every layer and is therefore very time-consuming.
There have been attempts for quite some time to eliminate at least one coating process to shorten the production time of the pattern.
EP 0 711 213 B1, for example, describes a further rapid prototyping process namely selective laser sintering. In this case the particulate material used is croning sand, that is hot-coated foundry sand with resol or novolack resin. This means that only the particulate material containing resin must be applied and the application of the binding agent is superfluous. Traditional foundry materials can also be used in this process and with these existing casting patterns can be produced from the usual materials familiar to the expert.
However, this production process also has considerable disadvantages. The resin in the sand is not completely hardened during the exposure process. This leads to a reduced so-called green part rigidity of the produced moulds. The desired rigidity is reached only after removal of the loose sand and a subsequent oven process. Apart from the additional process step in the oven there is a high risk of breakage while de-sanding and handling the “green parts”. During the oven process an undesired distortion of the components can also occur.
Furthermore croning sands have a relatively high thermic stability, which leads to a poor de-coring capability at the relatively low casting temperatures associated with light alloy casting.
For selective laser sintering croning sands with a higher binder content are also necessary. The consequence of this are larger quantities of gas during the pyrolysis of the binder while casting and therefore a higher reject risk due to blowholes in the component.
Moreover selective laser sintering in general has the disadvantage that the laser is a complex technique and in addition the exposure phase is also relatively time-consuming.
Furthermore only a very limited choice of sand types and grain sizes are available for selective laser sintering, which means that this process is also not very flexible.
A so-called 3D printing process is familiar from the patents U.S. Pat. No. 5,204,055 and also EP 0 431 924 B1. This entails the selective adhesion of particulate material by the addition of binding material. This process has an advantage over selective laser sintering in that it is based on a cost-effective printing technology.
It must be said that, because of the unfavourable material properties, typical traditional foundry binders can be administered only with great technical effort. There is also a danger that the nozzles used to measure out the binding agent become clogged and unusable.
Using a drop dispenser to administer the binding agent makes the mixing of the binder in the component very poor. In order to reach comparable rigidities as with conventionally mixed sands, much higher quantities of binder must be added, which again leads to problems in casting because of the higher quantities of gas.
In PCT/DE00/03324 a further 3D printing process is revealed. This is a selective printing of particles mixed with binder with an activator, to which a gas curing device is connected.
Again it is advantageous that here traditional foundry materials can be used.
However, gas curing is very elaborate for this process. Materials which create a health hazard such as SO2 are partly necessary, meaning that a very large amount of equipment is required and the safe operation of the apparatus becomes very costly.
As prior to the curing process not even the slightest solidification of the component takes place, slight displacement of the powder bed while layering can lead to the destruction of the entire component.
A further 3D printing process is familiar from DE 197 23 892 A1. This is a selective printing using particles coated with binder, so-called croning sand, with a moderating agent. This is again followed by curing which, according to the disclosure in this publication, takes place by way of radiation. This process also has the advantage of being able to use traditional foundry materials. However, curing of the components is very complex in this process too, because the necessary narrow tolerance in the change of temperature requires an extensive use of equipment.
In the process published in DE 198 53 834 A1, again a 3D printing process, a selective printing of particles sprayed with binder with curing agent takes place. Here too traditional foundry materials can again be used with some degree of flexibility.
The disadvantages of this process include the complex spray application of the binder, inhomogeneous binder mixing and the high concentration of binder in the component.
Moreover, due to the formation of mist in the building chamber caused by the spraying process, a high degree of soiling of the apparatus is consequently caused. As a result an elaborate cleaning of the print head is necessary, as otherwise a hardening of the material on the nozzles occurs and causes its destruction.
Similar disadvantages are illustrated in the selective printing of untreated sand with binding and curing agent, as described in WO 01/72502 A1.